1. Field of the Invention
The present invention relates to an electromagnetic interference (EMI) prevention apparatus, and more particularly to an EMI prevention apparatus for a flat panel display which is capable of minimizing EMI by a clock signal generated from a format converter of the flat panel display using a spread spectrum clocking method.
2. Description of the Related Art
It is common practice that most electric energy using devices generate a certain amount of electromagnetic noise, which is transferred in the form of electromagnetic radiation over the air or in the form of electromagnetic conduction over a conductor such act a power line.
Electromagnetic interference (EMI) is basically generated by three elements: a noise source for emitting electromagnetic energy in the form of electromagnetic conduction or radiation; a propagation medium for transferring the emitted electromagnetic energy; and damaged equipment subject to interference by noise. The EMI is not generated if any one of the three elements is excluded.
The EMI damages a variety of elements, such as communication equipment, control facilities, computer devices, human beings, etc. Effects of the EMI depend on a noise intensity, a transfer path, a distance from a noise source to damaged equipment, a coupled structure of the noise source and damaged equipment, and a resisting force of the damaged equipment.
Conventional EMI removal methods are generally adapted to reduce EMI by using a coaxial cable and a shielded line and mounting a shielding cover to given equipment, or by using a specific device capable of reducing the EMI.
However, such conventional methods are disadvantageous in that the use of additional elements for EMI reduction increases the cost and weight of given equipment.
In order to overcome the above problem, a spread spectrum clocking (SSC) scheme has been proposed for EMI reduction.
The SSC scheme is adapted to modulate the frequency of a clock signal according to a modulation profile of a predetermined frequency to increase the frequency of the clock signal and reduce the amplitude thereof, so as to reduce EMI.
FIG. 1 shows a modulation profile of a general spread spectrum clocking (SSC) method.
As shown in FIG. 1, according to the general SSC method, the frequency of a clock signal is modulated between a nominal frequency fnom 5 of a constant frequency clock signal and a down-spreading frequency (1-δ)fnom according to a modulation profile 9. Here, δ represents a spreading magnitude as a percentage of the nominal frequency fnom 5. The modulation profile 9 determines an energy distribution shape of a clock spectrum based on the spread spectrum clocking.
FIG. 2 is a block diagram of a general spread spectrum clocking (SSC) system, which is denoted by the reference numeral 100.
A description will hereinafter be given of the construction of the general SSC system 100 with reference to FIG. 2.
The general SSC system 100 comprises a first divider 110 for receiving a non-modulated input clock signal, a phase detector 120 for receiving an output signal from the first divider 110, which is a non-modulated input clock signal, and an input signal to a feedback divider 150, which is a frequency-modulated clock signal, measuring a phase difference between the non-modulated input clock signal and the frequency-modulated clock signal and providing the measured phase difference as its output signal, a charging pump 130 for receiving the output signal from the phase detector 120 and generating charges in response to the received signal, and a loop filter 140 for receiving the charges from the charging pump 130 and generating a direct current (DC) voltage output. The SSC system 100 further comprises the feedback divider 150, a voltage controlled oscillator (VCO) 160 and a post divider 170.
In the general SSC system 100 with the above-mentioned construction, the initial input clock signal, not modulated, is frequency-modulated according to a modulation profile. As a result, owing to the frequency modulation, the amplitude of the input clock signal is reduced while the frequency thereof is increased, thereby preventing EMI.
The SSC scheme as described above is applied to a computer system to increase the frequency of a clock signal for a high-speed process of a computer and reduce the amplitude thereof so as to reduce EMI, but it is not applied to a flat panel display.
In order to remove EMI with a flat panel display, there have conventionally been proposed a method for reducing the amount of electromagnetic energy conducted or radiated from a signal source itself, a method for reducing the amount of electromagnetic energy generated on a transfer path, and a method for protecting object equipment to be damaged, from EMI.
The first or third method may use a shielding plate to minimize EMI, and the second method may use a coaxial cable or a shielded conductor to minimize EMI.
However, the method using the shielding plate or coaxial cable is a passive method in that it interrupts radiated electromagnetic waves.
Provided that the amount of electromagnetic waves conducted or radiated from an EMI source can first be reduced, a necessity for using a separate shielding structure or device in a signal source, transfer path or object equipment will be removed.